During Release 12, the LTE standard has been extended with support of device to device (D2D) (specified as “sidelink”) features targeting both commercial and Public Safety applications. Some applications enabled by Rel-12 LTE are device discovery, where devices are able to sense the proximity of another device and associated application by broadcasting and detecting discovery messages that carry device and application identities. Another application consists of direct communication based on physical channels terminated directly between devices.
D2D communications may be extended to support Vehicle-to-X (V2X) communications, which includes any combination of direct communication between vehicles, pedestrian carried devices, and infrastructure mounted devices. V2x communication may take advantage of available network (NW) infrastructure, although at least basic V2x connectivity can be possible in case of lack of available network infrastructure. Providing an LTE-based V2x interface may be economically advantageous because of the LTE economies of scale and it may enable tighter integration between communications with the NW infrastructure (vehicle-to-infrastructure (V2I)), vehicle-to-pedestrian (V2P), and vehicle-to-vehicle (V2V) communications, as compared to using a dedicated V2x technology.
V2x communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements sets, e.g., in terms of latency, reliability, capacity, etc.
The European Telecommunications Standards Institute (ETSI) has defined two types of messages for road safety: Co-operative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM).
A CAM message is intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast fashion. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. The CAM message also serves as active assistance to safety driving for normal traffic. Devices check availability of a CAM message every 100 ms, yielding a maximum detection latency requirement is not more than 100 ms for most CAM messages. However, the latency requirement for Pre-crash sensing warning is not more than 50 ms.
A DENM message is event-triggered, such as by braking, and the availability of a DENM message is also checked for every 100 ms, and the requirement of maximum latency is not more than 100 ms.
The package size of CAM and DENM message can vary from more than 100 to more than 800 bytes, although the typical size is around 300 bytes depending on the specific V2X use case, message type (e.g. DENM can be larger than CAM), and depending on the security format included in the packet (e.g., full certificate or certificate digest). The message is supposed to be detected by all vehicles in proximity.
The Society of the Automotive Engineers (SAE) has defined a Basic Safety Message (BSM) for DSRC with various defined messages sizes. Based on the importance and urgency of the messages, the BSMs are further classified into different priorities.
Sensing-Based Resource Allocation with Booking
In V2x communications, two major types of traffic are distinguished: recurrent traffic and event-triggered traffic. Various embodiments disclosed herein are mostly related to recurrent traffic, where the transmitted packets arrive regularly (e.g., they may be strictly periodic or have some deviation from an average periodicity).
One efficient way to schedule recurrent-traffic V2x transmissions is to use radio resource booking. In resource allocation using resource booking a user equipment (UE) can book radio resources in advance for transmitting the next packet (including all the retransmissions). The minimum time span of a booking is usually taken to be the minimum time between two consecutive packets (e.g., the minimum message periodicity). Similarly, the maximum time span of a booking is usually taken to be the maximum time between two consecutive packets (e.g., the maximum message periodicity). For example, in V2X the time interval between the generation of two consecutive CAM messages may not be lower than 100 ms (in the absence of congestion control) and may not exceed 1 s. Thus, it is reasonable to allow bookings for 100 ms, 200 ms, . . . , or 1 s, as it is currently being considered by 3GPP. Usually, the UE signals the booking information to other UEs. This allows a receiving UE to predict the future utilization of the radio resources by reading received booking messages and schedule its current transmission to avoid using the same resources. To do so, a UE needs to sense the channel for some time duration preceding the (re)selection trigger to gather booking messages. In addition, it may also be possible to transmit unhooking messages that release previously booked resources. For accurate prediction, the sensing time should be long enough to detect booking and/or unhooking messages from other relevant UEs.
FIG. 1 illustrates an example of the sensing-based resource selection mechanism with booking. Let T be the minimum time between two recurrent transmissions by a UE, which is referred to as “basic period”. That is, a UE with recurrent traffic transmits, at most, one packet every T seconds (a transmission may consist of several retransmissions, although this is not illustrated in FIG. 1 for simplicity). In FIG. 1, UE 1 transmits a packet at time to and meanwhile books, e.g., transmits a booking message to other UEs indicating, its intention to transmit the next packet at ta+4T. Similarly, UE 2 transmits a packet at time tb and meanwhile books, e.g., transmits a booking message to other UEs indicating, its intention to transmit the next packet at tb+2T. At time tc, UE3 wants to select or reselect a radio resource for its transmission within the time window [tc,tc+T]. UE3 has been monitoring the channel during a time window of size 4T. UE3 uses its channel observations in this window to predict the future utilization of the radio resources and accordingly select a radio resource for its transmission (e.g., a resource that is not indicated by the above bookings to avoid potential collision).
It is clear that to achieve good performance the sensing window must be long enough to include as many bookings as possible/necessary. Commonly, the size of the sensing window is sufficiently large to roughly cover the longest possible booking (in terms of basic periods). In the example in FIG. 1, the sensing window is chosen to consist of 4 basic periods. In the remainder of this disclosure, the expressions “sensing over the entire window” and “sensing over the whole window” refer to performing the sensing operation using the largest possible window size (i.e., the largest window size that the system allows for).
It is noted that in this example and in the rest of this disclosure, UEs may or may not operate using a common division of the time in terms of basic periods. That is, time may be divided into “basic periods” in the same way for all UEs or, alternatively, different UEs may have different divisions of time into “basic periods”.
Problems with Existing Solutions
In the present disclosure, it is realized that in systems with long sensing windows, there is a large energy consumption associated with operating the UE to sense booking related message signaling from other UEs. In addition, large sensing windows may require the UE to perform quickly complex operations. This may be problematic for some types of UEs that are subject to restrictions on capabilities and/or energy, e.g., pedestrian carried or worn UEs.
One alternative with lower energy consumption and complexity that has been discussed in 3GPP is to perform sensing over only the last part of the time window, for example sensing over the last basic period, as illustrated in FIG. 2. Sensing over the last part of the time window, referred to in FIG. 2 as a reduced sensing window, of the whole/entire sensing window may not be able to detect all the relevant booking messages and/or unhooking messages transmitted by UEs since such signaling may not occur in the reduced sensing window. For example, in FIG. 2, UE3 cannot receive the booking message transmitted by UE 1 since the message is transmitted at time ta, where time ta is less than tc−T. Similarly, the sensing window of UE3 does not contain the time at which the booking message by UE2 is transmitted.
Some of the sensing-based resource allocation algorithms discussed in 3GPP make a prediction on the availability of radio resources by taking averages (or other operations) of measurements in the past. For example, to estimate the load level of a certain radio resource at a future time t+tc, the sensing takes the average of the measured load at times t+tc−T, t+tc−2T, t+tc−3T, etc. The UE may then control timing of its sensing based on the prediction, the effectiveness of which is affected by the quality of the estimates.
Thus, sensing using entire windows has a high associated energy consumption whereas sensing using less than the entire window, i.e. over only the last part of the time window, has an associated degradation in performance affecting all users in the system (due to collisions of transmissions).